docstr: exit the heartbeat server.
          
docstr_tok: ["exit","the","heartbeat","server","."]
code: def exit(self):
      """exit the heartbeat server."""
      self.exit_flag = True
  
imports: import grpc
  import os
  import time
  import threading
  from concurrent import futures
  from parl.remote import remote_constants
  from parl.remote.grpc_heartbeat import heartbeat_pb2
  from parl.remote.grpc_heartbeat import heartbeat_pb2_grpc
  from parl.utils import logger, get_ip_address
  import psutil
  import multiprocessing
label_desc: Use image node_id 3 for calling the GrpcHeartbeatServer obj's underlying member method code with example usage: obj.exit() without return types